The present invention relates to a frequency multiplier to be used as a high-stability, low-noise signal source for microwave and millimeter-wave communication systems.
As a constitution of the high-stability, low-noise signal source for microwave and millimeter-wave communication systems, there has been provided a technique that with a plurality of frequency multipliers connected in series on the output side of a PLL (phase-locked loop) oscillator that generates low frequency signals, an output frequency of the PLL oscillator is sequentially multiplied to obtain a signal of a desired frequency. In such a PLL oscillator, conventionally, it has been common practice to connect in series frequency multipliers that doubles the frequency.
An example of the frequency multiplier as a doubler is shown in "Monolithic Microwave Integrated Circuits (edited by the Institute of Electronics, Information and Communication Engineers), pp. 125-127". This frequency multiplier, as shown in FIG. 7, comprises an input-side matching circuit 101, an FET (Field Effect Transistor) 102, a transmission line 103, an end-open stub 104, and an output-side matching circuit 105.
In the frequency multiplier of this constitution, with the gate of the FET 102 biased to around the pinch-off, a large-amplitude input signal with frequency f0 is inputted to the gate of the FET 102 via the matching circuit 101. Then, the drain waveform of the FET 102 becomes close to a half-wave rectified one, which has a large portion of frequency components that are integral multiples of the input signal frequency f0, particularly even-ordered frequency components. On the drain side of the FET 102, a fundamental-wave trap circuit employing the end-open stub 104 is used to suppress the fundamental wave. Because the length of the end-open stub 104 is selected so as to be an electrical length of 90.degree. with respect to the fundamental wave of the input signal, the connecting point of the end-open stub 104 is short-circuited to the input signal fundamental wave and appears open to the doubled wave, so that fundamental-wave components are suppressed and the doubled wave is outputted. This frequency multiplier has an additional amplification effect by the FET 102 so that a high-efficiency frequency multiplication can be achieved.
Also, an example of frequency multipliers that quadruple the frequency at one stage with a view to reduction in the number of stages to be connected is shown in "A 15/60 ONE-STAGE MMIC FREQUENCY QUADRUPLER", IEEE, 1996 Microwave and Millimeter-Wave Monolithic Circuits Symposium pp. 35-38".
This frequency multiplier as a quadruplet, as shown in FIG. 8, comprises an input-side matching circuit 201, a transmission line 202, an end-open stub 203 having an electrical length of 90.degree. with respect to the doubled wave, an end-open stub 204 having an electrical length of 90.degree. with respect to the quadrupled wave, an HEMT (High Electron Mobility Transistor) 205, a transmission line 206, an end-open stub 207 having an electrical length of 90.degree. with respect to the doubled wave, an end-open stub 208 having an electrical length of 90.degree. with respect to the fundamental wave, and an output-side matching circuit 209. Because the end-open stub 208 having an electrical length of 90.degree. with respect to the fundamental wave and the end-open stub 207 having an electrical length of 90.degree. with respect to the doubled wave are connected to the circuit on the drain side of the HEMT 205, there appears no signals of the fundamental wave (frequency f0) and the doubled wave (frequency 2f0). These end-open stubs are open to the signal of the quadrupled wave (frequency 4f0), so that the signal of the quadrupled wave is outputted. Also, because the end-open stub 203 having an electrical length of 90.degree. with respect to the doubled wave and the end-open stub 204 having an electrical length of 90.degree. with respect to the quadrupled wave are connected to the input side, signals of the doubled wave (frequency 2f0) and the quadrupled wave (frequency 4f0) reflected from the HEMT 205 toward the input side are suppressed.
With regard to the frequency multiplier as a doubler, when frequency multipliers are connected in series in multiple stages to obtain a desired frequency, the number of high-frequency transistors (FETs) increases with increasing number of order of frequency multiplication. This causes problems of increased complexity of the circuit and increased power consumption.
Also, in frequency multipliers as doublers and frequency multipliers as quadruplers, at voltages higher than the pinch-off, the drain current of the FETs (including HEMTs) generally exhibits a square characteristic to the gate voltage. This causes a problem of poor efficiency at the time when harmonics of tripled wave or higher-ordered waves are taken out.
Therefore, an object of the present invention is to provide a frequency multiplier which is capable of taking out fourth- or higher-ordered harmonics efficiently with a simple constitution using one high frequency transistor, and which can be reduced in size and stabilized in operation.
In order to achieve the aforementioned object, there is provided a frequency multiplier comprising:
an input-side matching circuit;
a high frequency transistor with base or gate connected to an output terminal of the input-side matching circuit and with emitter or source grounded;
first signal transmission means with one end connected to collector or drain of the high frequency transistor;
doubled-wave reflection means with one end connected to the other end of the first signal transmission means;
second signal transmission means with one end connected to the other end of the first signal transmission means;
fundamental-wave reflection means with one end connected to the other end of the second signal transmission means; and
an output-side matching circuit with its input terminal connected to the other end of the second signal transmission means.
In an embodiment of the present invention, the first signal transmission means is a first transmission line;
the second signal transmission means is a second transmission line;
the doubled-wave reflection means is an end-open stub for blocking passage of a doubled wave; and
the fundamental-wave reflection means is an end-open stub for blocking passage of a fundamental wave.
According to this frequency multiplier, when an input signal is inputted to the base of the high frequency transistor (e.g., bipolar transistor) via the input-side matching circuit, the collector current of the high frequency transistor increases exponentially relative to the base voltage, so that many harmonics are outputted. Then, because a short-circuit to the fundamental wave is formed at the connecting point of the end-open stub for blocking the passage of the fundamental wave, the fundamental wave is reflected toward the high frequency transistor. Also, because a short-circuit to the doubled wave is formed at the connecting point of the end-open stub for blocking the passage of the doubled wave, the doubled wave is reflected toward the high frequency transistor. Thus, only harmonics of quadrupled wave or higher-ordered waves are outputted via the output-side matching circuit. When this occurs, the fundamental wave is reflected toward the high frequency transistor by the end-open stub for blocking the passage of the fundamental wave, so that the output power of the doubled wave is increased by the multiplying effect of the high frequency transistor. The doubled wave of increased output power is further reflected toward the high frequency transistor by the end-open stub for blocking the passage of the doubled wave, so that the output power of the quadrupled wave is further increased by the multiplying effect of the high frequency transistor. Also, in order that the fundamental wave is converted to the doubled wave to a maximum while the doubled wave is converted to the quadrupled wave to a maximum, the electrical lengths of the first transmission line and the second transmission line are optimized to, for example, about 10-40.degree. with respect to the fundamental wave, by which the output efficiency of the quadrupled wave is increased. Further, by appropriately setting the characteristic impedances of the end-open stub for blocking the passage of the doubled wave and the end-open stub for blocking the passage of the fundamental wave to, for example, about 20-70 .OMEGA., the doubled wave can be effectively suppressed without narrowing the frequency band of the quadrupled wave. Thus, the frequency multiplier is capable of taking out fourth- or higher-ordered harmonics efficiently with a simple constitution using one high frequency transistor, particularly taking out the quadrupled wave with high output efficiency. In addition, similar effects can be obtained also by using as the high frequency transistor a MESFET (MEtal Semiconductor Field Effect Transistor) or HEMT (High Electron Mobility Transistor) or the like.
In an embodiment of the present invention, the first signal transmission means is a first inductor;
the second signal transmission means is a second inductor;
the doubled-wave reflection means is a circuit generally equivalent to an end-open stub for blocking passage of a doubled wave, the circuit being an equivalent circuit for blocking passage of the doubled wave and being constituted by a concentrated constant circuit using an inductor and a capacitor; and
the fundamental-wave reflection means is a circuit generally equivalent to an end-open stub for blocking passage of a fundamental wave, the circuit being an equivalent circuit for blocking passage of the fundamental wave and being constituted by a concentrated constant circuit using an inductor and a capacitor.
According to this frequency multiplier, when an input signal is inputted to the base of the high frequency transistor (e.g., bipolar transistor) via the input-side matching circuit, the collector current of the high frequency transistor increases exponentially relative to the base voltage, so that many harmonics are outputted. Then, because a short-circuit to the fundamental wave is formed at the connecting point of the equivalent circuit for blocking the passage of the fundamental wave, the fundamental wave is reflected toward the high frequency transistor. Also, because a short-circuit to the doubled wave is formed at the connecting point of the equivalent circuit for blocking the passage of the doubled wave, the doubled wave is reflected toward the high frequency transistor. Thus, only harmonics of quadrupled wave or higher-ordered waves are outputted via the output-side matching circuit. When this occurs, the fundamental wave is reflected toward the high frequency transistor by the equivalent circuit for blocking the passage of the fundamental wave, so that the output power of the doubled wave is increased by the multiplying effect of the high frequency transistor. The doubled wave of increased output power is further reflected toward the high frequency transistor by the equivalent circuit for blocking the passage of the doubled wave, so that the output power of the quadrupled wave is further increased by the multiplying effect of the high frequency transistor. Also, in order that the fundamental wave is converted to the doubled wave to a maximum while the doubled wave is converted to the quadrupled wave to a maximum, the reactances of the first inductor and the second inductor are optimized, by which the output efficiency of the quadrupled wave is increased. Thus, the frequency multiplier is capable of taking out fourth- or higher-ordered harmonics efficiently with a simple constitution using one high frequency transistor, particularly taking out the quadrupled wave with high output efficiency. Furthermore, when the inductors are implemented by chip inductors or spiral inductors and the capacitors are implemented by chip capacitors or MIM capacitors, and when the distributed constant circuits of the end-open stubs are replaced with concentrated constant circuits using inductors and capacitors, the circuit occupancy area can be reduced. In this case, in particular, effects can be obtained when an MMIC is implemented by using spiral inductors and MIM capacitors. In addition, similar effects can be obtained also by using as the high frequency transistor a MESFET or HEMT or the like.
In an embodiment of the present invention, the emitter or the source of the high frequency transistor is grounded via a transmission line.
According to this frequency multiplier, by inserting a transmission line between the emitter (or source) of the high frequency transistor and the ground, the reflection coefficient on the input side of the high frequency transistor can be reduced, so that the input-side matching can be easily attained without significantly losing the multiplication gain. As a result of this, the reflection characteristics at the input end of this frequency multiplier can be improved and the circuit operation can be stabilized. It is noted that the electrical length of the transmission line is preferably 1-15.degree. with respect to the fundamental wave.
In an embodiment of the present invention, the emitter or the source of the high frequency transistor is grounded via an inductor.
According to this frequency multiplier, by inserting an inductor between the emitter (or source) of the high frequency transistor and the ground, the reflection coefficient on the input side of the high frequency transistor can be reduced, so that the input-side matching can be easily attained without significantly losing the multiplication gain. As a result of this, the reflection characteristics at the input end of this frequency multiplier can be improved and the circuit operation can be stabilized. It is noted that the reactance of the inductor is preferably 1-10 .OMEGA. with respect to the fundamental wave.
In an embodiment of the present invention, the high frequency transistor is a heterojunction bipolar transistor.
According to this frequency multiplier, in a comparison between the harmonic output characteristics of the HBT (Heterojunction Bipolar Transistor) and another high frequency transistor, for example, an HEMT, which are operated at around the pinch-off, the difference between fundamental wave and doubled wave is similar in level to each other, whereas the output power ratio of third- or higher-ordered harmonics is higher in the HBT than the HEMT. Thus, the output power of harmonics can be increased by employing the HBT as the high frequency transistor.